1. Field of the Invention
The present invention relates to a process for preparing an ester of an unsaturated carboxylic acid, in which an ester of the unsaturated carboxylic acid and a C1-C4-alkanol is reacted with an alcohol R3OH in the presence of a transesterification catalyst. The invention also relates to a metal alkoxide and the use of the metal alkoxide as transesterification catalyst in the process of the present invention.
2. Description of the Background
Among the esters of unsaturated carboxylic acids, (meth)acrylic esters are of particular industrial interest since they are valuable starting compounds for the preparation of polymers and copolymers which are employed, for example, as fibers, plastics, surface coatings, dispersions or adhesives. The term (meth)acrylic esters refers to methacrylic esters and acrylic esters. The present invention can be applied particularly advantageously to the preparation of these esters and is described below by way of example for (meth)acrylic esters. However, it can be applied generally to the transesterification of lower carboxylic esters, preferably α,β-unsaturated carboxylic esters, and particularly preferably to the transesterification of esters of unsaturated monocarboxylic acids having from 3 to 6 carbon atoms and unsaturated dicarboxylic acids having from 4 to 8 carbon atoms.
The transesterification of lower carboxylic esters with higher alcohols for preparing higher carboxylic esters in the presence of acidic or basic catalysts, in particular the preparation of (meth)acrylic esters by transesterification in the presence of acidic or basic catalysts, is generally known. In addition, it is generally known that the transesterification reaction is an equilibrium reaction. To achieve economically viable conversions, it is therefore necessary either for one of the starting materials to be used in a large excess or for at least one of the reaction products to be removed from the equilibrium, i.e. continually separated from the reaction mixture. Preference is given to separating off the product having the lowest boiling point, namely the lower alkanol liberated, which generally forms an azeotrope with the lower carboxylic ester. Accordingly, the transesterification is generally carried out by heating a mixture of the lower carboxylic ester, the higher alkanol, a catalyst and a polymerization inhibitor or inhibitor mixture to boiling and separating off the azeotrope of lower carboxylic ester and lower alkanol at the top of a distillation column which is generally located on top of the transesterification reactor.
A series of problems occur in such a case. Firstly, unsaturated carboxylic acids tend to polymerize under the action of heat or light. Particularly during the preparation and the purification by distillation, they are subjected to temperatures which can easily trigger an undesirable polymerization. The result is contamination of the apparatuses, blockage of lines and pumps and fouling of column trays and heat exchange surfaces. Cleaning of the plants is a laborious, expensive and environmentally unfriendly procedure which, in addition, greatly reduces the availability of the plants.
A further problem is the formation of by-products, e.g. of Michael addition products (addition of alkanol onto the carbon-carbon double bond), of ethers from primary alcohols and of olefins from secondary alcohols, as a result of dehydration. The secondary reactions firstly lead to losses of desired product and secondly make complicated separation and purification steps necessary.
Many of the known transesterification catalysts which are available lose activity over time. Some catalysts are so sensitive to hydrolysis that the reaction has to be carried out with careful exclusion of water, or else they decompose to form substances which require additional purification and separation steps.
Catalysts customary at present are, in particular, titanium alkoxides and zirconium alkoxides whose alkyl groups are C1-C4-alkyl radicals, e.g. tetramethyl, tetraethyl, tetraisopropyl, tetrapropyl, tetraisobutyl and tetrabutyl titanate (CH 522 584, CH 464 891, U.S. Pat. No. 2,822,348, EP 298 867, DE 2 319 688, GB 960 005, GB 1 012 817, GB 1 016 042, EP 145 588). However, these titanates and zirconates do not give satisfactory results since some of them are thermally unstable and extremely sensitive to hydrolysis and therefore easily lead to interfering impurities. In addition, they frequently have to be hydrolyzed to form insoluble products before they can be separated from the reaction mixture (GB 1 012 817, GB 1 016 042, U.S. Pat. No. 2,822,348).
The use of titanium tetraisopropoxide or tetrabutoxide, the most common transesterification catalysts, introduces, for example, isopropanol or butanol as impurities which generally accumulate in the unreacted lower ester which is distilled off. Owing to their boiling points and frequently also the formation of azeotropes, they are difficult to remove and can therefore lead to the formation of further by-products by means of transesterification reactions or addition onto the double bond of the esters.
Furthermore, it is known that alkyl titanates promote polymerization and can therefore give rise to formation of polymer during the preparation and work-up of the ester. This leads to a reduction in the yield, to fouling of apparatuses and to a shortening of plant running times.
Various proposals have been made for solving these problems. CH 464 891 A recommends, owing to the hydrolysis-sensitivity of the titanates, that the transesterification be carried out batchwise. In contrast to a continuous process, this enables the starting materials to be dewatered without an additional engineering outlay. However, it is disadvantageous for an industrial-scale preparation. EP 118 639 A proposes transesterification with the titanium alkoxide of the higher alkanol in the absence of the higher alkanol and in the absence of water in order to prevent the formation of azeotropes. In this process, the lower (meth)acrylic ester is reacted with the titanium alkoxide of the higher alkanol, forming the target ester together with the titanate of the lower alkanol which is, in a separate reaction step, reacted once more with the higher alkanol to form the corresponding titanate. The process is complicated and requires large amounts of titanate. It is therefore of no economic importance.
EP 298 867 A describes the transesterification of ethyl acrylate with dialkylaminoalkanols in the presence of tetraethyl titanate so as to avoid the introduction of an additional alkanol. However, methyl and ethyl titanates are expensive and, according to U.S. Pat. No. 3,686,268, are less suitable as catalysts. Owing to the thermal instability of titanates, GB 1 012 817 A proposes hydrolysis of the titanates at elevated temperature before isolation of the target ester by distillation. the formation of “lacquer deposits” in the distillation apparatus is said to be avoided in this way. Since the process includes a technically complicated filtration step and does not allow reuse of the catalyst, it is of no economic importance.
GB 1 016 042 A additionally proposes the use of a “plasticizer oil”. According to U.S. Pat. No. 2,822,348, the titanate catalyst is hydrolyzed before isolation of the product by distillation or remains in the product.
To substantially avoid polymer formation, various patents and patent applications have recommended the addition of various inhibitors or inhibitor mixtures, for example aminophenols and aminomethylphenols (EP 145 588), hydroquinone, hydroquinone monomethyl ether, phenothiazine, tert-butylcatechol, methylene blue, copper sulfate or iron sulfate, alone or in admixture (EP 298 867), hydroquinone, hydroquinone monomethyl ether, phenothiazine, etc., optionally with addition of oxygen or air (CH 464 891, U.S. Pat. No. 5,037,978).
To avoid polymerization, DE 1 067 806 A proposes carrying out the transesterification under superatmospheric pressure at 180-250° C. to achieve very short residence times. Such reaction conditions require complicated apparatuses and are therefore uneconomical.
DE 1 067 805 A describes the use of quinhydrone as inhibitor and the addition of part of the lower acrylic ester required for complete transesterification only during the transesterification via the column located on top of the reactor. The process is cumbersome and requires a 3-4-fold excess of lower acrylic esters.
EP 522 709 A proposes a polymerization inhibitor composition comprising a plurality of components including an N,N′-dinitrosophenylenediamine compound, a phenothiazine and, if desired, additionally a hydroquinone, a hydroquinone monomethyl ether and/or a phenylenediamine compound.
U.S. Pat. No. 5,171,888 proposes a combination of a phenylenediamine compound and a manganese compound as polymerization inhibitor.
A further way of avoiding polymeric by-products and also for avoiding Michael additions and formation of ethers and olefins from alkanols is described in U.S. Pat. No. 5,037,978 and DE 2 805 702. U.S. Pat. No. 5,037,978 uses hafnium chelate compounds as catalysts and DE 2 805 702 uses chelate compounds of zirconium and/or calcium as catalysts.
U.S. Pat. No. 3,686,268 describes the use of titanium phenoxides as catalysts. These catalysts do not introduce any undesirable alkanols, are said to be stable to hydrolysis and reusable and additionally have a stabilizing action. The titanium phenoxides employed in U.S. Pat. No. 3,686,268 reduce the risk of polymerization, but not to an extent sufficient for the industrial transesterification of (meth)acrylic esters. They are therefore not used industrially. There is therefore a need for still further improved catalysts and processes for preparing higher esters of unsaturated carboxylic acids from their lower esters.